The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries. The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). At the same time, mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
In addition, heightened awareness concerning toxic waste has motivated, in part, efforts to replace toxic cadmium electrodes in nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is a strong market potential for environmentally benign secondary battery technologies.
Secondary batteries are in widespread use in modern society, particularly in applications where large amounts of energy are not required. However, it is desirable to use batteries in applications requiring considerable power, and much effort has been expended in developing batteries suitable for high specific energy, medium power applications, such as, for electric vehicles and load leveling. Of course, such batteries are also suitable for use in lower power applications such as cameras or portable recording devices.
At this time, the most common secondary batteries are probably the lead-acid batteries used in automobiles. Those batteries have the advantage of being capable of operating for many charge cycles without significant loss of performance. However, such batteries have a low energy to weight ratio. Similar limitations are found in most other systems, such as Ni-Cd and nickel metal hydride systems.
Among the factors leading to the successful development of high specific energy batteries, is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-toxic, and easy to process.
Thus, a smaller, lighter, cheaper, non-toxic battery is sought for the next generation of batteries. The low equivalent weight of lithium renders it attractive as a battery electrode component for improving weight ratios. Lithium provides also greater energy per volume than do the traditional battery standards, nickel and cadmium.
The low equivalent weight and low cost of sulfur and its nontoxicity renders it also an attractive candidate battery component. Successful lithium/organosulfur battery cells are known. (See, De Jonghe et al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No. 5,162,175.)
However, employing a positive electrode based on elemental sulfur in an alkali metal-sulfur battery system has been considered problematic. Although theoretically the reduction of sulfur to an alkali metal sulfide confers a large specific energy, sulfur is known to be an excellent insulator, and problems using it as an electrode have been noted. Such problems referred to by those in the art include the necessity of adjoining the sulfur to an inert electronic conductor, very low percentages of utilization of the bulk material, poor reversibility, and the formation of an insulating sulfur film on the carbon particles and current collector surface that electronically isolates the rest of the electrode components. (DeGott, P., "Polymere Carbone-Soufre Synth ese et Propri etes Electrochimiques," Doctoral Thesis at the Institut National Polytechnique de Grenoble (date of defense of thesis: 19 Jun. 1986) at page 117.)
Similarly, Rauh et al., "A Lithium/Dissolved Sulfur Battery with an Organic Electrolyte," J., Electrochem. Soc., 126 (4): 523 (April 1979) state at page 523: "Both S.sub.8 and its ultimate discharge product, Li.sub.2 S, are electrical insulators. Thus it is likely that insulation of the positive electrode material . . . led to the poor results for Li/S cells."
Further, Peramunage and Licht, "A Solid Sulfur Cathode for Aqueous Batteries," Science, 261: 1029 (20 Aug. 1993) state at page 1030: "At low (room) temperatures, elemental sulfur is a highly insoluble, insulating solid and is not expected to be a useful positive electrode material." However, Peramunage and Licht found that interfacing sulfur with an aqueous sulfur-saturated polysulfide solution converts it from an insulator to an ionic conductor.
The use of sulfur and/or polysulfide electrodes in non-aqueous or aqueous liquid-electrolyte lithium batteries (that is, in liquid formats) is known. For example, Peled and Yamin, U.S. Pat. No. 4,410,609, describe the use of a polysulfide positive electrode Li.sub.2 S.sub.x made by the direct reaction of Li and S in tetrahydrofuran (THF). Poor cycling efficiency typically occurs in such a cell because of the use of a liquid electrolyte with lithium metal foil, and the Peled and Yamin patent describes the system for primary batteries. Rauh et al., "Rechargeable Lithium-Sulfur Battery"(Extended Abstract), J. Power Sources, 26: 269 (1989) also notes the poor cycling efficiency of such cells and states at page 270 that "most cells failed as a result of lithium depletion."
Other references to lithium-sulfur battery systems in liquid formats include the following: Yamin et al., "Lithium Sulfur Battery,"J. Electrochem. Soc., 135(5): 1045 (May 1988); Yamin and Peled, "Electrochemistry of a Nonaqueous Lithium/Sulfur Cell," J.Power Sources, 9: 281 (1983); Peled et al., "Lithium-Sulfur Battery: Evaluation of Dioxolane-Based Electrolytes,"J. Electrochem. Soc., 136(6): 1621 (June 1989); Bennett et al., U.S. Pat. No. 4,469,761; Farrington and Roth, U.S. Pat. No. 3,953,231; Nole and Moss, U.S. Pat. No. 3,532,543; Lauck, H., U.S. Pat. Nos. 3,915,743 and 3,907,591; Societe des Accumulateurs Fixes et de Traction, "Lithium-sulfur battery," Chem. Abstracts, 66: Abstract No. 111055d at page 10360 (1967); and Lauck, H. "Electric storage battery with negative lithium electrode and positive sulfur electrode," Chem. Abstracts, 80: Abstract No. 9855 at pages 466-467 (1974).)
DeGott, Supra, notes at page 118 that alkali metal-sulfur battery systems have been studied in different formats, and then presents the problems with each of the studied formats. For example, he notes that an "all liquid" system had been rapidly abandoned for a number of reasons including among others, problems of corrosiveness of liquid lithium and sulfur, of lithium dissolving into the electrolyte provoking self-discharge of the system, and that lithium sulfide forming in the positive (electrode) reacts with the sulfur to give polysulfides Li.sub.2 S.sub.x that are soluble in the electrolyte.
In regard to alkali metal-sulfur systems wherein the electrodes are molten or dissolved, and the electrolyte is solid, which function in exemplary temperature ranges of 130.degree. C. to 180.degree. C. and 300.degree. C. to 350.degree. C., DeGott states at page 118 that such batteries have problems, such as, progressive diminution of the cell's capacity, appearance of electronic conductivity in the electrolyte, and problems of safety and corrosion. DeGott then lists problems encountered with alkali metal-sulfur battery systems wherein the electrodes are solid and the electrolyte is an organic liquid, and by extension wherein the negative electrode is solid, the electrolyte is solid, and the positive electrode is liquid. Such problems include incomplete reduction of sulfur, mediocre reversibility, weak maximum specific power (performance limited to slow discharge regimes), destruction of the passivating layer of Li.sub.2 S as a result of its reaction with dissolved sulfur leading to the formation of soluble polysulfides, and problems with the stability of the solvent in the presence of lithium.
DeGott also describes on page 117 a fundamental barrier to good reversibility as follows. As alkali metal sulfides are ionic conductors, they permit, to the degree that a current collector is adjacent to sulfur, the propagation of a reduction reaction. By contrast, their reoxidation leads to the formation of an insulating sulfur layer on the positive electrode that ionic ally insulates the rest of the composite, resulting in poor reversibility.
DeGott concludes on page 119 that it is clear that whatever format is adopted for an alkali metal-sulfur battery system that the insulating character of sulfur is a major obstacle that is difficult to overcome. He then describes preliminary electrochemical experiments with a composite sulfur electrode prepared from a slurry. The slurry was prepared by mixing the following components in acetonitrile: 46% sulfur; 16% acetylene black; and 38% (PEO).sub.8 /LiClO.sub.4 (polyethylene oxide/lithium perchlorate). The resulting slurry was then deposited on a stainless steel substrate by "capillary action." From those preliminary experiments, DeGott concludes on page 128 that it is clear that, even when optimizing the efficiency of the composite electrode (that is, by multiplying the triple point contacts) that elemental sulfur cannot be considered to constitute an electrode for a secondary battery, in an "all solid" format.
Present solid-state lithium secondary battery systems are limited to a specific energy of about 120 Wh/kg. It would be highly desirable to have a battery system characterized by higher specific energy values.
It would be even more desirable if solid-state batteries having practical specific energy values greater than about 150 Wh/kg could operate at room temperature. It would be additionally advantageous if solid-state batteries having high specific energy and operation at room temperature could be reliably fabricated into units with reproducible performance values.
In lithium cells wherein a liquid electrolyte is used, leakage of the electrolyte can leave lithium exposed to the air, where it rapidly reacts with water vapor and oxygen. Substantial casing can prevent such reactions and protect users and the environment from exposure to hazardous, corrosive, flammable or toxic solvents but adds unwanted weight to the battery. A solid-state battery would greatly reduce such problems of electrolyte leakage and exposure of lithium, and would allow reducing the weight of the battery.
Furthermore, a battery formulation that overcomes the problem of lithium depletion described in the prior art, for example, Rauh et al., supra, would have many advantages.
In summary, disadvantages in currently available metal-sulfur battery systems include poor cycling efficiency, poor reversibility, lithium depletion, or operating temperatures above 200.degree. C., among other problems. Practitioners in the battery art have long sought a solid-state or gel-state metal-sulfur battery system that would overcome these limitations.